Lithographic techniques are widely practiced in the manufacture of microelectronic devices. These techniques include the use of a "resist" or a layer of a material which is deposited on a substrate, delineated into a specific pattern, and used as a mask for etching. As microelectronic devices become increasingly three-dimensional due to miniaturization, single level resists with an imageable layer are no longer sufficient for lithography because of step coverage and resolution problems. In an attempt to solve these problems, trilayer resists have been used, and are especially suitable for the lithographic definition of small features, i.e., features smaller than about 2 .mu.m.
Typically, trilayer resists include an underlying layer deposited directly on the substrate to be processed. Because the substrate typically does not have a planar surface, this layer is deposited with a thickness sufficient to present an essentially planar surface, and accordingly is referred to as a "planarizing" layer. An intermediate protective layer, such as a silicon oxide layer, typically is then formed on the planarizing layer. This intermediate layer acts to protect a subsequently formed pattern from degradation during processing. Lastly, a third imageable layer is formed on the intermediate layer. The imageable layer is typically a photosensitive material that is delineable by exposure to radiation to create a pattern, for example by using a mask. The imageable layer is subsequently developed so that the mask pattern is transferred to the imageable layer.
Although trilayer resists provide a technique for producing fine features during semiconductor fabrication, the use of trilayer resists requires several discrete processing steps, can be cumbersome and require large capital investments. Accordingly, bilayer resists, which eliminate one layer, for example by incorporating the protective intermediate layer with the imaging layer, have been heavily investigated by researchers.
Bilayer resists are less cumbersome than trilayer resists for imaging sub-micron features on semiconductor devices which have surface topography requiring a planarizing layer. Typically bilayer resists comprise (1) a lower level that contacts the surface of the substrate and (2) an upper layer overlying the lower layer that is patterned, i.e., lithographically defined. To be effective, advantageously the lower layer is susceptible to removal by contact with oxygen reactive ion etching ("RIE") and the upper layer is essentially unaffected by RIE relative to the underlying level. Despite the enormous amount of research conducted in this area, however, very few bilayer resists have been developed that offer both desirable resolution (i.e., better than about 0.2 .mu.m) and a suitable resistance to plasma etching (for example, a resistance to oxygen RIE of greater than 20 times greater than that of the planarizing layer, a ratio desirable for pattern reproducibility).
In bilayer resists, silicon can be incorporated into the resist polymer to provide oxygen RIE resistance. The etch rate of the polymer decreases as the weight percent of silicon increases. Incorporating silicon into polymers, however, also generally decreases the temperature at which sub-micron features are dimensionally stable, measured, for example, by the decrease in glass transition temperature ("T.sub.g ") of the polymer.
U.S. Pat. No. 4,061,799 to Brewer discloses the use of poly(styrene)-poly(butadiene) block copolymers as negative tone electron beam resists, i.e. a resist that cross links upon irradiation and thus becoming insoluble in many developers. The resist, however, offers insufficient oxygen RIE resistance and insufficient resolution for use in bilayer resist schemes.
U.S. Pat. No. 4,892,617 to Bates et. al. discloses the use of poly(dimethylsiloxane)-poly(chlorinatedmethylstyrene) block copolymers as negative tone electron beam resists. These resists, however, offer insufficient contrast and polymerization techniques used can result in undesirable side reactions.
M. Jurek and E. Reichmanis, Polymers in Microlithography, pp. 158-174 (1989) report the use of phenolic resin-dimethyl siloxane block copolymers. These copolymers, however, provide limited resolution and RIE protection.
M. Bowden et. al., Makromol. Chem., Macromol. Symp. 53, 125-137 (1992) disclose the use of poly(1-butene sulfone-graft poly(dimethylsiloxane) copolymers as positive tone electron beam resists. The resists, however, can suffer poor shelf life inherent in poly(1-butene sulfone) resists.
Although these processes can be used to produce resists that have good resolution, the processes can generate undesirable side reactions and/or lack sufficient protection against etching. Accordingly, there exists a need for a bilayer resist which provides desirable resolution, sufficient etching protection yet also exhibits dimensional stability.